1. Field of the Invention
The present invention relates to a magnetic memory from and/or to which information is read and/or written using magnetism, and in particular, to an information reading-writing technology suitable for reversing magnetization utilizing electron spins.
2. Description of the Related Art
In recent years, MRAM (Magnetic Random Access Memory) has been receiving attention as a storage device for use in information processing devices such as computers and communication devices. With MRAM, data is stored magnetically, and the direction of magnetization can be maintained without using any electrical means. Volatile memory such as DRAM (Dynamic Random Access Memory) and SRAM (Static RAM) has the disadvantage that information is lost when power failure occurs. However, such a disadvantage can be avoided with MRAM. Furthermore, when compared with conventional non-volatile storage means such as flash EEPROM and hard disk devices, MRAM is superior in terms of access speed, reliability, power consumption, and the like. Therefore, it is believed that MRAM has functions which can replace all the functions of volatile memories such as DRAM and SRAM and of nonvolatile storage means such as flash EEPROM and hard disk devices.
For example, in the development of information devices with the aim of realizing so-called ubiquitous computing which enables information processing at any location, storage devices are required which are adaptable to high-speed processing while power consumption is reduced, and in which loss of information can be avoided even when power failure occurs. MRAM has the potential to meet all these requirements simultaneously and is expected to be employed in a variety of information devices in the future.
In particular, sufficient power supply cannot always be provided in tablets, portable information terminals, and the like which are intended to be carried on a daily basis. Therefore, in order to carry out a large amount of information processing in severe use environments, a further reduction in power consumption during information processing is required even for MRAM, for which low power consumption is expected.
Some examples of the technique for further reducing power consumption in MRAM, being magnetic memories of a spin injection magnetization reversing type are described in, for example, some articles of Nikkei Electronics, 2003, pp. 98-105; Nikkei Microdevices, 2004, pp. 85-87; “Spin-torque transfer in batch-fabricated spin-valve magnetic nanojunctions,” J. Z. Sun, D. J. Monsma, T. S. Kuan, M. J. Rooks, D. W. Abraham, B. Oezyilmaz, A. D. Kent, and R. H. Koch, J. Appl. Phys., Vol. 93, No. 10, Parts 2&3, 15 May 2003; “Spin Pumping in Ferromagnetic-Metal/Normal-Metal Junctions, S. Mizukami,” Y. Ando and T. Miyazaki, Journal of Magnetics Society of Japan, Vol. 27, No. 9, 2003, and the like. In the spin injection magnetization reversing type magnetic memory, a current is applied to a main body of a magnetic storage element exhibiting, for example, a tunneling magneto-resistive (TMR) effect in order to directly reverse the magnetization of a free layer by utilizing the spin torque of the electrons. Hence, magnetization reversal is not strongly affected by a demagnetizing field and the like even when the size of the magnetic storage element is reduced, and therefore writing can be performed with a small current.
As shown in FIG. 10, a magnetic memory 500 has a magnetic storage element 501 that is disposed in each storage area (memory cell), and a read-write line 510 that is provided at the opposite ends of the magnetic storage element 501.
The magnetic storage element 501 includes: a first magnetic layer (free layer) 502 in which the direction of magnetization can be reversed; a second magnetic layer (fixed layer) 504 in which the direction of magnetization is fixed; and a non-magnetic layer 506 sandwiched between the first magnetic layer 502 and the second magnetic layer 504. An antiferromagnetic layer 508 is stacked on the outer side of the second magnetic layer 504, and the direction of magnetization of the second magnetic layer 504 is pinned in the direction of an arrow K by the antiferromagnetic layer 508.
As shown in FIG. 11(A), when a write current is applied to the magnetic storage element 501 in the direction shown by the arrow A via the read-write line 510, spin-polarized electrons e with spin oriented in the same direction as the fixed magnetization direction K of the second magnetic layer 504 pass through the second magnetic layer 504 and are injected into the first magnetic layer 502. When the current value exceeds a certain critical value, these electrons e cause a magnetization reversal such that the magnetization direction G of the first magnetic layer 502 is oriented in the same direction as the magnetization direction K of the second magnetic layer 504.
Meanwhile, when a write current is applied to the magnetic storage element 501 in the direction of the arrow B, as shown in FIG. 11(B), the electrons e pass through the first magnetic layer 502. Among these electrons, electrons e with spin oriented opposite to the fixed magnetization direction K of the second magnetic layer 504 are reflected away from the boundary between the non-magnetic layer 506 and the second magnetic layer 504. However, electrons e with spin oriented in the same direction as the fixed magnetization direction K pass through the second magnetic layer 504 and flow out of the magnetic storage element 501. Therefore, the electrons e that are spin-polarized in the opposite direction to the fixed magnetization direction K are concentrated in the first magnetic layer 502. Hence, these electrons e cause a magnetization reversal such that the magnetization direction G of the first magnetic layer 502 is oriented in the opposite direction to the magnetization direction K of the second magnetic layer 504.
In the magnetic storage element 501, the resistance value is different depending on whether the direction of magnetization of the first magnetic layer 502 is parallel or antiparallel to that of the second magnetic layer 504. The resistance value of the magnetic storage element 501 can be detected by applying a read current to the read-write line 510, and a binary value can be read by utilizing the difference between the states shown in FIGS. 11(A) and (B).
In the magnetic memory 500, the magnetization state of the first magnetic layer 502 can be directly reversed by applying a write current to the magnetic storage element 501. Hence, the write current value can be reduced.
However, in the structure of the magnetic memory 500 of the spin injection magnetization reversal type, both a read current and a write current must be applied to the magnetic storage element 501 through the same passage. Therefore, the difference (margin) between the read and write currents must be set to be a large value by increasing the write current value and decreasing the read current value. This is because when the margin is too small, even the read current may cause accidental writing. Specifically, the read current value must be set to one half or less, and desirably one tenth or less of the write current value.
For example, when the write current is set to 1 mA, the read current must be reduced to approximately 100 μA. Furthermore, when the write current is reduced to 100 μA, as may be possible in the near future, the read current will therefore need to be set to approximately 10 μA. However, since it is difficult to increase the read sensitivity of the magnetic storage element 501, it is difficult to correctly detect the resistance value of the magnetic storage element 501 by applying a weak read current.
Therefore, although the reduction of the write current can be achieved technically, the write current must be set to a relatively large value in order to prevent accidental writing (magnetization reversal of the free layer) during the read operation. Hence, there is a limit to the reduction in current consumption.